Asynchronous transfer mode (ATM) networks carry fixed sized cells within the network irrespective of the applications being carried over ATM. At the network edge or at the end equipment, an ATM Adaptation Layer (AAL) maps the services offered by the ATM network to the services required by the application. There are a number of industry standards and proposed standards covering various AALs. In particular, "B-ISDN ATM Adaptation Layer Type 2 Specification," draft Recommendation 1.363.2, November 1996, of ITU-T (herein referred to as AAL-2) provides for efficient ATM transport of small, delay-sensitive packets in such applications as packet voice systems.
In such a packet voice system, audio is transmitted in packets. Silences are suppressed (i.e., durations when signal power is below a predefined threshold). A sequence of audio packets, starting from the end of a silence duration to the beginning of the next silence duration, is referred to herein as a "talk-spurt." A transmitter provides each packet with a sequence number. The range of sequence numbers is typically finite and repeats. For example, there may be eight sequence numbers, 0-7. At the start of the first talk-spurt, the first transmitted voice packet includes the sequence number 0. After the first eight packets are transmitted the sequence numbers begin to repeat, starting again at 0. During silent intervals, the transmitter still counts sequence numbers such that the packet at the start of the next talk-spurt receives a sequence number just as if the silence interval had, instead, contained voice packets.
Upon receiving the first packet of a call, the receiver waits for an initial period of time, referred to herein as the "build-out" delay, before reconstructing and playing out the audio signal during a connection, or call. Once the build-out delay has passed, the receiver reconstructs the audio signal using the recovered sequence numbers to re-order received packets for the duration of the connection. Unfortunately, the use of sequence numbers, by themselves, and a single build-out delay for the entire call does not mitigate other anomalies present in packet voice systems due to packet delay and packet loss.